The invention relates to a method for making a semiconductor device and more particularly to making a semiconductor device using a metal oxide.
One area of interest in the semiconductor technology is the development of high-K dielectrics for use as a gate dielectric. This is important due to the limitations encountered as the conventional gate dielectric material is scaled to improve performance. Silicon dioxide has been the preferred choice for gate dielectric because of its good electrical properties and compatibility with the conventional materials. As the silicon dioxide thickness is scaled, and becomes very thin, the leakage current becomes high and results in unacceptably high power consumption. To reduce the power consumption, it is desirable to replace the silicon dioxide with a dielectric material that is physically thicker but that has a higher dielectric constant to achieve the same effective silicon dioxide thickness.
Metal oxides are such high K materials that are being studied for this purpose. In a conventional CMOS process flow, it is important to be able to selectively remove the metal oxide from the source/drain regions. One of the difficulties with metal oxides, including zirconium oxide, is that they etch differently than many of the conventional materials commonly used in the semiconductor industry. For example, hydrofluoric acid (HF) is the conventional etch for silicon dioxide but has not proven to be effective for removing some metal oxide materials. Often some of the metal oxide is not completely removed, and the selectivity to silicon oxide is poor. A long HF etch time will remove substantial amounts of the field oxide and is very undesirable. Furthermore, some of the commonly used chemistries such as hydrochloric acid (HC1), SC1 (NH40H:H202:H20), sulfuric acid (H2S04), and piranha (H2S04:H202) were found to be ineffective for etching zirconium oxide.
Another concern in removing a metal oxide is that the underlying silicon in the active regions must remain relatively smooth. An issue with chemicals sometimes used for metal oxide etching is preferential etching along grain boundaries. This forms openings in the metal oxide and results in etching through the interfacial oxide that exists in the area immediately above and adjacent to the substrate. The problem is that the etchant disrupts and makes the surface of the exposed underlying silicon rough. A rough silicon surface is very undesirable. It increases the source drain resistance and affects the source drain junction depths. It is important that the source/drain junction depths be consistent from one transistor to another. Thus, there is a need for a metal oxide etch that does not cause excessive roughness of the silicon and does not take an excessive amount of time in order to etch the metal oxide.